voltammetric determination of nitrate in water samples at copper modified...

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Voltammetric determination of nitratein water samples at copper modifiedbismuth bulk electrodeDawei Pan a , Wenjing Lu a b , Haiyun Zhang a b , Li Zhang a b &Jianmei Zhuang ca Key Laboratory of Coastal Zone Environmental Processes , YantaiInstitute of Coastal Zone Research(YIC), Chinese Academy ofSciences(CAS); Shandong Provincial Key Laboratory of CoastalZone Environmental Processes, YICCAS , Yantai Shandong ,264003 , P.R. Chinab Graduate School of the Chinese Academy of Sciences , Beijing,100039 , P.R. Chinac The Key Lab in Molecular and Nano-materials Probes of theMinistry of Education of China, College of Chemistry, ChemicalEngineering and Materials Science , Shandong Normal University ,Jinan 250014 , P.R. ChinaPublished online: 14 Jun 2012.

To cite this article: Dawei Pan , Wenjing Lu , Haiyun Zhang , Li Zhang & Jianmei Zhuang (2013)Voltammetric determination of nitrate in water samples at copper modified bismuth bulkelectrode, International Journal of Environmental Analytical Chemistry, 93:9, 935-945, DOI:10.1080/03067319.2012.690149

To link to this article: http://dx.doi.org/10.1080/03067319.2012.690149

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Voltammetric determination of nitrate in water samples at coppermodified bismuth bulk electrode

Dawei Pana*, Wenjing Luab, Haiyun Zhangab, Li Zhangab and Jianmei Zhuangc

aKey Laboratory of Coastal Zone Environmental Processes, Yantai Institute of Coastal ZoneResearch(YIC), Chinese Academy of Sciences(CAS); Shandong Provincial Key Laboratory ofCoastal Zone Environmental Processes, YICCAS, Yantai Shandong, 264003, P.R. China;

bGraduate School of the Chinese Academy of Sciences, Beijing, 100039, P.R. China;cThe Key Lab in Molecular and Nano-materials Probes of the Ministry of Education of China,

College of Chemistry, Chemical Engineering and Materials Science,Shandong Normal University, Jinan 250014, P.R. China

(Received 2 September 2011; final version received 11 April 2012)

Voltammetric determination of nitrate (NO�3 ) in 0.1M Na2SO4 solution (pH 2.0)at copper modified bismuth bulk (BiB/Cu) electrode was presented in this article.Owing to the unique properties of bismuth bulk electrode, the proposed BiB/Cuelectrode can be used for determination of NO�3 without any deoxygenatingpretreatment and shows low detection limit (6 mM), wide linear range (13 mM to3mM), high sensitivity (31.8 mAmM�1 cm�2) and good anti-interference. Thepractical application of the proposed BiB/Cu electrode has been carried out fordetermination of NO�3 in real environmental water samples.

Keywords: bismuth bulk electrode; copper; nitrate; voltammetric determination;water samples

1. Introduction

Nitrate (NO�3 ) has a widespread distribution within environmental, food, industrial andphysiological systems. Lots of studies have proved that excessive NO�3 can present bothenvironmental and physiological problems [1,2]. Nitrate pollution in groundwater, riversor lakes is now a common and increasing problem in numerous worldwide countries.Owing to the possible dangers associated with NO�3 , the World Health Organization(WHO) recommends a maximum limit of 0.8mM for NO�3 concentration in drinkingwater [3]. The US Environmental Protection Agency (EPA) has also imposed a limit of0.7mM NO�3 on drinking water. So it is clear that monitoring NO

�3 concentration in

aqueous solutions is extremely important.Different techniques such as UV/vis, chemiluminescence, fluorimetry, infrared, Raman

and molecular cavity emission, chromatography, capillary electrophoresis and electro-chemistry have been used for the quantification determination of NO�3 . It should bepointed out that the majority of the procedures depend on a previous step involving thestoichiometric conversion of NO�3 to nitrite (NO

�2 ) by using convenient chemical reductors

such as Cu/Cd [4].

*Corresponding author. Email: [email protected]

Intern. J. Environ. Anal. Chem., 2013Vol. 93, No. 9, – , http://dx.doi.org/10.1080/03067319.2012.6

© 2013 Taylor & Francis

935 945 90149

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Electrochemical techniques has great potential for on-site environmental monitoringdue to its favourable portability, suitability for automation, short analysis time, low powerconsumption and inexpensive equipment. The studies in the electroanalytical field on thedetermination of NO�3 are almost based on voltammetric/amperometric or potentiometricmethods. The latter involves the use of ion-selective electrodes [5–8]. Comparatively,voltammetric/amperometric techniques depend on the electrochemical reduction of NO�3at electrode surfaces. Various substrate materials such as copper [9–14], nickel [15],copper–nickel alloys [16], copper–palladium alloys [17], platinum [18,19], lead [20], silver[21], rhodium [22], boron-doped diamond [23–25], gold [26,27] and integrated electronictongue [28] have been used. But, owing to slow kinetics of charge transfer, direct reductionof NO�3 on bare unmodified electrodes is usually difficult. The surface cumulativepassivation effects of bare electrodes will also lead to low sensitivity and irreproducibility[13]. Additionally, oxygen reduction occurs at a more positive potential than nitratereduction, the large current from the oxygen reduction interferes with the nitratemeasurement. Direct reduction of NO�3 on many electrodes is usually in deoxygenatedsolution [12,29–31]. Thus, new alternative electrode materials for voltammetric analysisare still highly desired to meet the growing demands for on-site environmental monitoringof NO�3 .

Recently, metallic bismuth as an attractive new electrode material for eitherelectrochemical or electroanalytical use has been reported [32–35]. Bismuth bulk (BiB)electrode possesses several advantages: besides being a non-toxic metal, it exhibits arelatively high hydrogen overvoltage resulting in wide pH dependent cathodic potentialwindow approaching that of mercury. The anodic stripping voltammetry performance ofthe BiB electrode shows characteristics that are similar to those of the bismuth filmelectrode. Bi-based electrodes are less susceptible to oxygen interference according tostudies, where measurements at Bi-based electrodes were performed in non-deaeratedsolutions, yielding a flat baseline [36].

In this article, BiB electrode has been used as a new substrate electrode forvoltammetric determination of NO�3 for the first time. To improve the selectivity andsensitivity in the process of quantifying nitrate, the electrodeposition of copper ispretreated on the surface of BiB electrode, because copper is an effective material forelectroanalytical determination of NO�3 and the deposition of copper is both inexpensiveand simple. The proposed copper modified bismuth bulk (BiB/Cu) electrode can be usedfor voltammetric determination of NO�3 without any deoxygenating pretreatment andshows wide linear range and high sensitivity to NO�3 .

2. Experimental

2.1 Reagents

Bi rods (1.5 cm in length) were purchased from Sichuan University, China. The workingBiB electrode was made from a Bi rod encapsulated in an insulating epoxy housing thatwas equipped with a copper wire to make the electrical contact. CuSO4, H2SO4, Na2SO4,NaNO2 and NaNO3 were obtained from Sinopharm Chemical Reagent Co., Ltd(Shanghai, China). 0.1M stock standard of NO�3 was prepared by dissolving NaNO3into deionized water, and then diluted to various concentrations of working solutions.The supporting electrolyte (pH 2.0) in all experiments was a 0.1M Na2SO4 solutioncontaining 5mM H2SO4. All other chemicals were analytical reagents and used without

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further purification. Deionized water (18.2M� cm specific resistance) obtained with a PallCascada laboratory water system was used throughout.

2.2 Apparatus

All electrochemical experiments were carried out in a conventional three-electrode cellcontrolled by a CHI 660D Electrochemical Work Station (CH Instruments, Inc). A BiB(1mm in diameter) electrode was used as working electrode. A Cu bulk disc electrode(3mm in diameter) was also used as working electrode for comparison. A platinum foilwas applied as the counter electrode, and a saturated calomel electrode (SCE) served as thereference electrode. All potential values given below refer to SCE. The pH measurementswere performed at an E-201-C Model pH meter (Shanghai Leici Instrument Factory). Allthe electrochemical experiments were carried out at room temperature and non-deoxygenating condition.

The morphology and elemental composition of the modified electrodes werecharacterized by field emission scanning electron microscopy (SEM, Hitachi S-4800)and energy dispersive X-ray microanalyzer (EDS, HORIBA EX-350), respectively. Theion analysis was carried out by using a Dionex ICS3000 ion chromatography (USA).

2.3 Sample preparation

The laboratorial tap water was collected from the current authors’ institute. The well waterand underground water samples were obtained from Laishan district, Yantai city. Thelaboratorial tap water was used without any pretreatment. The well water andunderground water samples were first filtered through a standard 0.45 mm filter. Allwater samples were mixed with supporting electrolyte and acidified to pH 2. The watersamples were stored at �4�C in acid-cleaned high-density polyethylene (HDPE) samplebottles.

2.4 Analytical procedure

The procedure for preparation of BiB/Cu electrode is based on several pieces of research[12,13,19] and optimized as follows: (1) the BiB electrode was polished with 0.3 and0.05mm alumina slurries and washed with water and acetone, then activated in supportingelectrolyte by cyclic sweeping from �0.5V to 0.5V at a scan rate of 50mV s�1. (2) Copperwas electrodeposited on the BiB electrode by potentiostatic method from 0.1M CuSO4aqueous solution at �0.5V for 20 s.

The analysis of NO�3 was performed in 20mL beaker containing supporting electrolytesolution without removal of oxygen. Quantitative determinations of NO�3 were performedwith differential pulse cathodic voltammetry. The optimal conditions were as follows:increase of 0.01V; amplitude of 0.05V; pulse width of 0.2 s; pulse period of 0.5 s; quiet timeof 15 s. After each measurement, the BiB/Cu electrode can be used continuously aftercleaning with deionized water. Almost 20 times continuous measurement later, the BiB/Cuelectrode was cut to obtain a renewable Bi surface and regenerated for the followingexperiment according to the above procedure.

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3. Results and discussion

3.1 Characterization of the BiB and BiB/Cu electrodes

Figure 1 shows the morphology of the BiB and the BiB/Cu electrodes. From Figure 1(a),

a porous mesh-like structure can be observed for the BiB electrode. After the electro-

deposition of Cu, the different aspect can be obtained. The surface of BiB electrode was

partially covered by the dendrimer-like structure of Cu (Figure 1b). At the same time, the

presence of Cu on the surface of BiB electrode is also confirmed by EDS investigation.

From Figure 1(c), C, O and Bi are the major elements. C and O may come from the

treatment of Bi rod. The existence of Cu element on Figure 1(d) indicates that Cu exists

actually on the BiB electrode surface. It is well known that Bi forms binary or multi-

Figure 1. SEM images (a, b) and EDS pattern (c, d) of BiB (a, c) and BiB/Cu (b, d) electrodes.

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component ‘‘fusing’’ alloy with any other metals [37]. Therefore, it can be inferred that the

BiB electrode is essential to obtain Cu 3D electrodeposit and allows the strong

accumulation of Cu at the electrode surface and results in better response to NO�3 .The voltammetric behaviors of BiB (solid line) and BiB/Cu (dotted line) electrode in

0.1M Na2SO4 solution (pH 2.0) were shown in Figure 2. From the inset of Figure 2, two

oxidation peaks at �0.02V and 0.09V and one reduction peak at �0.26V can beobviously observed for BiB electrode, which correspond to the redox procedure of Bi.

Comparatively, a huge sharp peak at 0.22V can be obtained after electro-deposition of Cu

on BiB electrode (dotted line in Figure 2), which is in accordance with the oxidation of Cu.

Only one oxidation peak at 0.0V and one reduction peak at �0.25V of Bi were also seen.The another oxidation peak of Bi was suppressed in the huge oxidation peak of Cu. These

indicate the Cu particles were easily electro-deposited on BiB electrode surface, but not

covered the whole BiB electrode surface. This interesting Bi–Cu coexisting situation on

electrode surface will keep the good anti-interference to dissolved oxygen of Bi and

excellent electrocatalysis property of Cu, resulting in better voltammetric determination

of NO�3 .The electrochemical behaviour of the BiB and BiB/Cu electrode in 0.1M Na2SO4

solution (pH 2.0) with or without 100 mM NO�3 was shown in Figure 3. From Figure 3, noreduction peak of NO�3 can be found at BiB electrode in 0.1M Na2SO4 solution (pH 2.0),which shows that the direct reduction of NO�3 at Bi electrode is difficult. But at the BiB/Cuelectrode, an obvious reduction peak at �0.4V can be observed. These phenomena werecoincidence with the reported literature [13,29]. Additionally, the electrochemical

behaviour of NO�3 at the unpretreated Cu bulk electrode was also shown (the inset inFigure 3). Compared with the BiB/Cu electrode, no reduction peak of NO�3 can beobserved at unpretreated Cu bulk electrode, which is in accordance with the report [10].

This is may be the reason that the reduction of dissolved oxygen at unpretreated Cu bulk

E / V– 0.6 – 0.4 – 0.2 0.0 .2 .4 .6

i / µ

A

– 010

0

10

20

30

40

50

60

70

E / V– 0.6 – 0.4 – 0.2 0.0 .2 .4 .6

i / µ

A

– 04

– 02

0

2

Figure 2. Cyclic voltammograms recorded at the BiB (solid line) and BiB/Cu (dotted line) electrodesin 0.1M Na2SO4 solution (pH 2.0). Scan rate, 50mV s

�1.


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electrode caused the large background current and resulted in the disappear of thereduction peak of low concentration of NO�3 . The BiB electrode is essential to obtainactive Cu electrodeposit and the resulting BiB/Cu electrode is contributed to nitratereduction.

On the other hand, the reduction of NO�3 at BiB/Cu electrode in 0.1M Na2SO4solution (pH 2.0) with or without deoxygenating pretreatment was investigated and thecorresponding results are shown in Figure 4. Two distinct reduction peaks of NO�3 andNO�2 can be observed at BiB/Cu electrode in no deoxygenating solution, which is similarto those at the copper electrode preteated by dissolution/redeposition [10]. However, onlythe reduction peak of NO�3 can be found at BiB/Cu electrode in deoxygenating solution.As far as the electrode reactions are concerned, the thermodynamically preferred productof NO�2 reduction is N2, but other products such as NO, N2O, hydroxylamine andammonium are also formed [38]. The transformation of product in deoxygenating solutionmay cause the shift of the reduction peak of NO�2 . More interesting, the peak currents ofNO�3 are 0.272mA in deoxygenating solution and 0.279 mA in ambient solution. Almostsame peak currents indicate that the BiB/Cu electrode can be used for the determination ofnitrate without deoxygenating sample solutions because the existence of Bi might slow thereduction of dissolved oxygen.

3.2 Calibration curve

The mechanism of nitrate reduction at solid electrodes is to be resolved gradually andwould appear to be dependent upon the experimental conditions employed. However, inmost cases the final product of the electroreduction reaction at copper is ammonium in

E / V – 0.7 – 0.6 – 0.5 – 0.4 – 0.3 – 0.2

i / µ

A

– 0.30

– 0.25

– 0.20

– 0.15

– 0.10

– 0.05

0.00

E / V

i / µ

A

– 0.56

– 0.48

– 0.40

– 0.32

– 0.24

– 0.7 – 0.6 – 0.5 – 0.4 – 0.3 – 0.2

Figure 3. Differential pulse voltammograms recorded at BiB (dashed line, dashed and dotted line),BiB/Cu electrode (dotted line, solid line) and Cu bulk electrode (dotted line and solid line in Inset) in0.1M Na2SO4 solution (pH 2.0) with (dashed line, solid line) and without (dashed and dotted line,dotted line) 100mM NO�3 . Amplitude, 0.05V; Pulse width, 0.2 s; Sampling width, 0.0167 s; Pulseperiod, 0.5 s; Quiet time, 15 s.

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acidic solution [39–40] and nitrite in alkaline solution [21]. NO�3 reduction in sufficientacidic conditions can occur as an eight-electron process according to the followingequation [13]:

NO�3 þ 10Hþ þ 8e� ! 3H2OþNHþ4 ð1Þ

As shown in Equation 1, an acidic medium is required for the cathodic process. However,at very low pH values the onset of hydrogen evolution may interfere with the NO�3 electro-reduction as both processes occur at similar potentials. The optimized experiments wereperformed at pH 2.0. The calibration curve for NO�3 at the BiB/Cu electrode was derivedfrom the differential pulse voltammetric curves (Figure 5). The peak current (ip) isproportional to the concentration of NO�3 from 13 mM to 3mM (ip¼ 0.012þ 0.25C,r¼ 0.9994, ip in mA, C in mM). The sensitivity of the BiB/Cu electrode to NO�3 is31.8mAmM�1 cm�2. The detection limit of NO�3 calculated at S=N ¼ 3 is 6 mM. It is foundthat the BiB/Cu electrode shows higher sensitivity, wider linear range and lower detectionlimit than many other works [9–13,41]. The BiB/Cu electrode has also good reproducibilityand repeatability. The relative standard deviations for 100 mM NO�3 were found to be5.6% for one electrode with five measurements and 8.2% for five electrodes prepared withidentical procedures. The long-time stability of proposed electrode in deionized water wasperformed in supporting solution containing 200 mM NO�3 for three determinations ineach day and the corresponding result shows that almost 10% decrease of the originalcurrent response to NO�3 is observed in the first continuous 7 days.

E / V

– 0.7 – 0.6 – 0.5 – 0.4 – 0.3 – 0.2 – 0.1

i / µ

A

– 0.40

– 0.32

– 0.24

– 0.16

– 0.08

0.00

Figure 4. Cyclic voltammograms of the BiB/Cu electrode in 0.1M Na2SO4 solution (pH 2.0)containing 0.5mM NO�3 without (solid line) and with (dotted line) deoxygenating pretreatment.Scan rate, 50mV s�1.


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3.3 Study of interferences

The interference of common anions (Cl� and NO�2 ) to the detection of NO�3 was

investigated and the results are shown in Figure 6. In this case, the scan was initiated at�0.15V which may induces the partial stripping of copper from the electrode surface. Thepresence of Cl� in the electrolyte solution serves to scavenge some of the stripped copperand the reduction of NO�3 at copper electrodes is known to be dependent upon Cl

�

concentration [12]. The differential pulse voltammetric curves obtained at the BiB/Cuelectrode before and after the addition of Cl� were compared (Figure 6a). It can be seenthat although Cl� serves to enhance the NO�3 reduction peak, it also shifts the peaktowards more negative potentials. The presence of chloride serves to stabilize the Cuþ state

and the reduction of chloro-copper species occurs at more negative potentials. Thisbehaviour is consistent with previous report [12]. On the other hand, the reduction of NO�2was also investigated at the BiB/Cu electrode in order to evaluate the possibility ofsimultaneous detection of NO�3 and NO

�2 . Accordingly, Figure 6(b) shows voltammetric

curves recorded with the BiB/Cu electrode in solution containing NO�3 in the presence andabsence of NO�2 . The peak at �0.1V attributed to the reduction of NO�2 can be observedat the BiB/Cu electrode in solution without NO�2 , which imply the reduction of NO

�3

accompanies with the reduction of NO�2 [12]. At the same time, the peak of reduction ofNO�2 does not interfere with the peak of NO

�2 . The potential difference between NO

�3 and

NO�2 is almost 250mV. With the increase of the concentration of NO�2 , the peak currents

of NO�3 do not change significantly, while the peak currents of NO�2 increase obviously.

The good resolution between both cathodic peaks makes it possible further studiesinvolving speciation between NO�3 and NO

�2 at some particular samples.

C / mM

0.0 0.5 1.0 1.5 2.0 2.5 3.0

i p /

µA

0.0

0.2

0.4

0.6

0.8 y=0.012+0.25Xr=0.9994

E / V

– 0.5 – 0.4 – 0.3 – 0.2i /

µA

– 0.06

– 0.05

– 0.04

– 0.03

– 0.02

– 0.01

E / V– 0.6 – 0.5 – 0.4 – 0.3 – 0.2

i / µ

A

– 1.0

– 0.8

– 0.6

– 0.4

– 0.2

0.0

Figure 5. Calibration curves of NO�3 at BiB/Cu electrode. The values from bottom to top are 0,13, 25, 38, 50, 75, 100 mM for the left inset curves and 0.9, 1.1, 1.5, 2.0, 2.5, 3.0mM for theright inset curves respectively, which are in the linear range. The conditions are the same as inFigure 3.

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3.4 Practical application of BiB/Cu electrode

The BiB/Cu electrode exhibits high sensitivity, wide linear range and good interference for

the determination of NO�3 under the optimum experimental conditions. In order toillustrate its accuracy in practical analysis, the comparison between the proposed BiB/Cuelectrode and ion chromatography for detection of NO�3 in real samples was carried out.Several different types of environmental water samples including laboratorial tap water,

underground waters and well water were filtered through a standard 0.45mm filter andanalyzed by the standard addition method. The results are shown in Table 1. It can be seen

E / V

i / µ

A

– 0.20

– 0.15

– 0.10

– 0.05

0.00

E / V– 0.75 – 0.60 – 0.45 – 0.30 – 0.15 0.00

– 0.75 – 0.60 – 0.45 – 0.30 – 0.15 0.00

i / µ

A

– 0.20

– 0.16

– 0.12

– 0.08

– 0.04

0.00

(a)

(b)

Figure 6. Differential pulse voltammograms recorded at BiB/Cu electrode in 0.1MNa2SO4þ 0.5mM NO�3 solution (pH 2.0) before and after addition of Cl� (a) and NO�2 (b). Theconcentration of Cl� (from left to right) and NO�2 (from bottom to top) are both 1.5, 3 and 5mM.The conditions are the same as in Figure 3.


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that the BiB/Cu electrode indeed has a great potential for real sample analysis with a highaccuracy and good reliability.

4. Conclusions

A simple, inexpensive method for direct determination of NO�3 based on the Cu modifiedsolid BiB electrode has been demonstrated. BiB electrode is less susceptible to oxygeninterference and the proposed BiB/Cu electrode is tolerant of a range of potentialinterferences (Cl� and NO�2 ) that can significantly influence rival the determination ofNO�3 . The BiB/Cu electrode has lower detection limit, higher sensitivity and wide linearrange, which is suitable for a number of analytically relevant applications and particularlythose requiring water quality measurements. BiB can be used as a new alternative electrodematerial to provide an excellent platform for electroanalysis of anions.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China(21007087), the Chinese Academy of Sciences (KZCX2-YW-JS208), the Natural Science Foundationof Shandong Province (2008ZRA06004, BS2010HZ030), the Science and Technology DevelopmentProject of Yantai City (2009164), the Taishan Scholar Program of Shandong Province and theYouth Innovation Promotion Association of CAS.

References

[1] D. Reyter, D. Bélanger, and L. Roué, Electrochim. Acta 53, 5977 (2008).[2] R. Desai, M.M. Villalba, N.S. Lawrence, and J. Davis, Electroanalysis 21, 789 (2009).[3] World Health Organization, Rolling Revision of the WHO Guidelines for Drinking-Waters

Quality, Nitrates and Nitrites in Drinking-Waters. World Health Organization (2004).[4] M.J. Moorcroft, J. Davis, and R.G. Compton, Talanta 54, 785 (2001).[5] A.R. Asghari, M.K. Amini, H.R. Mansour, and M. Salavati-Niasari, Talanta 61, 557 (2003).

[6] J. Gallardo, S. Alegret, and M. Del Valle, Sens. Actuators B 101, 72 (2004).[7] T.A. Bendikov, J. Kim, and T.C. Harmon, Sens. Actuators B 106, 512 (2005).[8] S.S.M. Hassan, S.A.M. Marzouk, and H.E.M. Sayour, Talanta 59, 1237 (2003).

Table 1. Comparison of the BiB/Cu electrode and ion chromato-graphy for determination of NO�3 in real water samples.

Sample

Detected byBiB/Cuelectrodea

(mM)

Detectedby ion

chromatography(mM)

Laboratorial tap water 1.06� 0.08 1.10Underground water 1 0.18� 0.02 0.16Underground water 2 0.52� 0.09 0.58Underground water 3 0.19� 0.03 0.19Well water 2.30� 0.14 2.39

Note: aAverage value of three determinations� standard deviation.

D. Pan et al.944

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voltammetric determination of nitrate in water samples at copper modified...

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